Process for the production of a light sensitive body having an insulating photoconductive layer



United States Patent U.S. Cl. 117201 3 Claims ABSTRACT OF THE DISCLOSUREProcess for producing a light sensitive body having an insulatingphoto-conductive layer which comprises dispersing a photo-conductivecadium sulfide powder having an average grain size of less than 1.5microns in an electrically insulating thermo-setting resin having aspecific resistance (by volume) of at least 32 cm. at room temperature.The dispersion is applied to a support and heated to thus harden or curethe resin and to simultaneously heat treat the fine cadmium sulfidepowder and the resin.

This invention relates to a process for the production of a lightsensitive body and more particularly, to a process for the production ofa hard, highly sensitive, insulating photoconductive layer suitable foruse in electrophotography.

Some photoconductive inorganic compounds are well known to be useful asan insulating photoconductive layer for electrophotography. For example,there are metals, such as, sulfur and selenium, and metal compounds,such as, oxides, sulfides and selenides of zinc, cadmium, mercury,antimony, bismuth and lead, titanium oxide. These materials are formedin layers on a base plate, in particular, a metal plate or paper. Insome cases, insulating photoconductive layers are formed by dispersingthese materials in an electrically insulating film forming binder. Themost widely known and practically used of these materials are seleniumand zinc oxide. The other materials have not been put to practical useyet and no literature has been disclosed with respect thereto. In orderto obtain electrophotographic images using these materials, anelectrostatic charge is first given to the surface of the coatingthereof in a dark chamber, for example, by corona discharge, whereby toimpart light sensitivity thereto, and then the insulatingphotoconductive layer is exposed to light by an ordinary photographicmethod.

The electrostatic latent image obtained by the exposure to light isdeveloped by applying a fine powder of colored resin having electriccharges. The fine powder of colored resin is fixed on the insulatingphotoconductive layer, as it is or after transfer to a suitable support,by means of heat or solvent. When a transfer is carried out, theresidual fine powder on the light sensitive body is removed and thelight sensitive body is reused.

Selenium, which is widely known and practically used, is coated on ametal plate, in particular, an aluminum plate to form a layer of glassyselenium. This is used as a light sensitive body to be used repeatedly.The sensitivity thereof is highest, corresponding to ASA 2-10. Theglassy selenium is suitable for repeated uses, since its hardness ishigh and its surface is mirror-like. On the other hand, zinc oxide, theother material which is widely known and practically used, is dispersedin an electrically insulating resin binder and applied to a paper,followed by drying, whereby an electrophotographic sensitive body,called light sensitive paper, is formed. This light sensitive paper hasa 3,508,961 Patented Apr. 28, 1970 "ice sensitivity of ASA -0.01-0.2.Images are directly formed and fixed on the paper. Because of this thecolor is substantially white. This zinc oxide dispersed layer may beapplied to a metal base, but the resulting light sensitive body isunfavorably compared with that of selenium, the hardness of the coatingand the sensitivity being lower, and therefore, it is unfit for repeateduses. Moreover, the characteristics, in addition to the lower hardnessand sensitivity, deteriorate once it is exposed to light. Other thanselenium and zinc oxide, only titanium oxide has been noted lately forits practical use, and the others are only described in the literature.The selenium light sensitive body which can be used repeatedly exhibitsexcellent characteristics of light sensitivity, hardness of the lightsensitive layer and effect of pre-exposure to light, but is accompaniedby the following disadvantages. That is to say, the production of thematerial is very difficult. The selenium used as an insulatingphotoconductive layer for electrophotography is glassy and is formedinto a uniform film on an aluminum plate by the vapor deposition invacuum. The plant for its production should be of a large scale andprovided with means for vacuum vapor deposition. The efficiency of suchproduction is low. Another disadvantage lies in that the control ofadditives to be added for the purpose of enhancing the sensitivity andimproving other characteristics is very difficult due to the vapordeposition in vacuum. Furthermore, the glassy selenium is of one solidphase of selenium in a supercooled state. This tends to be crystallizedby the influences of temperature, moisture in the air and othermaterials, so the deterioration of its characteristics is promoted athigh temperature and humidity, resulting in shortening of its usefullife. In the case of zinc oxide, coloring matters are added for thesensitization, but this causes such defects that the length of life ismuch shorter than the selenium light sensitive body, since thesecoloring matters are thermally weak and a relatively rapid deteriorationof the characteristics is brought about by corona discharge and repeatedexposure to light.

In order to overcome the foregoing disadvantages, that is, difficultiesin production, low resistance to heat and contamination in the seleniumlight sensitive body, low sensitivity, low hardness of the lightsensitive layer, the effect of pre-exposure to light and instability tothermal shock in the zinc oxide light sensitive body, the presentinvention provides a highly insulating photoconductive layer, inparticular, electrophotographic light sensitive body, which can bereadily produced, has a light sensitivity which is the same as or morethan that of the selenium light sensitive body, is very stablethermally, is substantially free from the effect of pre-exposure tolight, is sufficiently resistant to repeated uses and has excellenthardness in the light sensitive layer.

The electrophotographic light sensitive body of this invention has beendeveloped with the objects described above and a process for theproduction thereof consists in dispersing a fine powder ofphotoconductive cadmium sulfide, having an average grain size of lessthan 1.5 micron, in a solvent solution of thermosetting resin having aspecific resistance by volume of at least 10 9 cm., applying theresulting dispersion to a suitable support, such as metal, and heatingthe support, thus coated, whereby to promote the hardening of the resinand to effect the heat treatment of the cadmium sulfide powder andresin.

The insulating photoconductive layer means a layer of such material thatdoes not or scarcely exhibits electric conductivity in a dark place, butexhibits or increases it upon exposure to light or other radiations. Amethod for the discrimination of the insulating photoconductive layer ofthis invention is, for example, to impart electrostatic charges to afree surface side of the insulating photoconductive layer formed on anelectrically conductive support in a dark place by means of coronadischarge. In this case, the electrostatic charges imparted can bemaintained for a considerably long time in a dark place. The polarity ofthe charge exhibits a difference in the rectifying property and lightsensitivity and differs by a material used. A selenium-type lightsensitive body is generally positively charged (plus) and a zincoxidetype light sensitive body is generally negatively charged (minus).The longer the time for maintaining electrostatic charges in a darkplace, the greater is the electrically insulating property. This ispreferable. In general, a time required for electric charges on asurface to decrease half of it initial value is several tens tothousands minutes. When a suitable light or radiant ray is radiated uponan insulating photoconductive layer having electric charges on thesurface, the electric conductivity is increased and the surface chargesare rapidly discharged. The attenuation velocity of surface chargesduring the radiation of light is as follows: It is 0.1-0.2 second forselenium-type light sensitive bodies and 2-20 seconds for zincoxide-type light sensitive bodies, though being different by the degreeof dye sensitization, as represented by a time required for the surfacecharges to decrease half of its initial value in radiating by the use ofa tungsten filament lamp of 100 lux. When light is radiated upon asurface of light sensitive body prior to charging it, in some cases, thekeeping capacity of electrostatic charges in a dark place is lowered andthe attenuation of charges in radiating light is accelerated orretarded. This is called the effect of pre-exposure to light in general,which must be avoided if it is desired to obtain stable imagesconstantly.

An object of the present invention is to provide a light sensitive bodywhich has the same capacity for keeping an electrostatic charge in adark place and the same attenuation velocity of surface charges inradiating light as a. selenium light sensitive body.

Another object of this invention is to provide an electrophotographiclight sensitive body capable of being used repeatedly to give stableimages substantially free from the effect of pre-exposure to light.

A still further object of our invention is to provide anelectrophotographic light sensitive body which is very stable thermally,has a high strength of the coating and long life and can be usedcontinuously repeatedly.

A still further object of our invention is to provide anelectrophotographic light sensitive body which can be produced in a verysimple manner with low cost and on a large scale.

A still further object of our invention is to provide an insulatingphotoconductive layer, the electric resistance of which is very high orhardly represents electric conductivity in a dark place, but theelectric resistance of which decreases or represents electricconductivity during the exposure to light or other radiant energy.

The insulating photoconductive layer of the electrophotographic lightsensitive body in accordance with the invention can be adapted,therefore, not only for electrophotography, but for means for convertingsignals of light or other radiant energy into electric signals, forexample, in a photocell, image intensifier and image converter.

As cadmium sulfide is a most excellent photoconductive material as iswell known, but has a low electric resistance in a dark place ingeneral, that is, 40 9 cm., this being much less than the 10 40 9 cm.required for an. insulating photoconductive material inelectrophotography, it has not been put to practical use yet in spite ofits possibilities. The studies on cadmium sulfide have been mainlyconcentrated in sintered bodies, in the production of large singlecrystals and in the production of sintered bodies from which theunevenness of electric properties among gains is eliminated as far aspossible. This results in practical use in photocells wherein thecharacteristics of cadmium sulfide are adequately utilized.

On the contrary, our efforts have been directed to reduction of the izesof crystals or primary grains, that is, to enlargement of surface areasthereof, and thereby to introduction of the unevenness of electricproperties at the" contacts among grains or the electric barriers,whereby the electric resistance in a dark place is increased. Althoughit remains unknown whether such reasoning is correct or not, anelectrophotographic light sensitive body which can be favorably put topractical use is obtained by dispersing a fine powder of photoconductivecadmium sulfide having an average grain size of less than 1.5 micron ina synthetic resin binder, applying the dispersion to a support anddrying to give an insulating photoconductive layer of resin-finelydivided photoconductive material dispersion. In this case, of course,the proportion of the binder and fine powder of cadmium sulfide shouldbe strictly selected, since the electric conductivity of the insulatingphotoconductive layer is controlled by the contacts among grains. It isfound that suitable proportions of the binder and cadmium sulfide iswithin -40% by volume of the binder to 2060% volume of cadmium sulfide,preferably within 6858% by volume of the former to 3242% by volume ofthe latter. This varies with the binder. As the binder to be used, asynthetic resin having a specific resistance by volume of more than 10 9cm., is preferably selected so as to maintain the electric resistance ofthe insulating photoconductive layer desired.

EXAMPLE 1 A dispersed solution having the following composition wasprepared:

Parts by wt. Copolymer of n-butyl methacrylate and i-butyl Thisdispersed solution was applied to a paper and dried under the followingconditions: Drying temperature-40 C.; drying tirne20 hours; coatingthicknessabout 30 microns.

Good results were obtained when this light sensitive paper wasnegatively charged (minus). The charging by corona discharge at 7.0 kv.gave a surface potential of 600-650 v. The attenuation velocity of thesurface potential in a dark place wa 8l0 minutes as represented by thetime required for the surface potential to decrease half of its valueand in radiating light of 20 lux by the use of tungsten-filament lamp,1.5 seconds are represented by the same. Images were obtained by such anexposure to light as in chlorobromide printing paper for silver saltphotography.

In the Example 1, an electrophotographic light sensitive paper wasshown, but the light sensitive paper could be reused by transferring theresulting images to another paper, followed by cleaning. In addition tothe synthetic resin shown in Example 1, cellulose type resins,polyesters, phenol resins, epoxy resins, amino resins, polyurethaneresins, silicone resins and copolymer of vinyl acetate vinyl chloridemay be used.

The electrophotographic light sensitive paper of the Example 1 has a lowcoating hardness and is not suitable for repeated uses, for the supportis of paper.

It is required for repeated uses that a metal sheet, metal pipe or otherelastic or flexible materials be used as a support and that themechanical strengths of the insulating photoconductive layer, forexample, the resistance to abrasion and furthermore, the resistance tochemicals and weathering, be excellent. To this end, the use of abinding agent of thermosetting type resin is recommended. Illustrativeof the thermosetting resins are phenol resins, xylene resins, urearesins, alkyd resins, unsaturated polyester resins, furan resins,silicone resins, epoxy resins and acrylic resins.

The invention is further illustrated by the following examples:

EXAMPLE 2 An elcctrophotographic light sensitive body was prepared bymixing a thermosetting acrylic resin, as a thermosetting resin, andcadmium sulfide, as a photoconductive material, to give a composition asshown in Table 1, the average grain size of the cadmium sulfide being0.2 micron, applying the resulting dispersion to an aluminum sheet andsubjecting it to a heat treatment at 150 C. for minutes. In Table 1 areshown the surface potentials after 2 seconds from the charge by coronadischarge in a dark place and the time required for the surfacepotential to decrease by half when light of 8 lux was radiated upon theelectrophotographic light sensitive body charged by a tungsten-filamentlamp. The thickness of the coating was approximately 30 microns afterdrying.

It is evident from the data of Table 1 that the allowable range in theproportions of a binder and cadmium sulfide powder is less than that ofthe zinc oxide type sensitive bodies in the prior art. The mostexcellent dispersions is D- corresponding to the composition ratio byvolume of 35.5 cadmium sulfide to 64.5 binder. D32- 37 showcharacteristics suitable for use as insulating photoconductive bodiesfor electrophotography. These light sensitive bodies are very excellentin lustre, hardness, adherence between the insulating photoconductivebody and aluminum sheet and resistance to wearing, and can be usedrepeatedly.

TABLE 1.COl\IPOSIIION AND CHARACTERISTICS OF MATERIALS OF EXAMPLE 2 TimeCdS Magicron 2 Surface pofor half- Dispersion N0. powder (g.) l 200clear (g.) tential volt value, see.

1 Cadmium sulfide powder: photoconductive powder, average grain size 0.2micron.

1 Thermosetting acryl resin lacquer (Kansai Paint Co., Ltd.).

EXAMPLE 3 A cylindrical light sensitive body was prepared by ap plyingthe dispersion of D-35 of the Example 2 to an aluminum pipe by sprayingand subjecting it to a heat treatment at C. for 30 minutes. The lightsensitive body presented a lustrous surface like enamel. The thicknessof the coating was approximately 35 micron after hardening.

EXAMPLE 4 A dispersed solution having the following composition wasapplied to an aluminum foil and subjected to a heat treatment at 150 C.for 60 minutes to produce a light sensitive body.

Parts by wt. Million No. 1 M clear 1 65 Photoconductive cadmium sulfide(used in Example 2) 75 Million No. 1 M thinner 120 Total 260 Epoxy resintype paint vehicle produced by Kansai Paint Co., Ltd.

6 EXAMPLE 5 ple 2) 100 Xylene 200 Total 348 Produced by Shin-EtsuChemical Industry 00., Ltd.

EXAMPLE 6 A dispersed solution having the following composition wasapplied to an iron pipe and subjected to heat treatment at 150 C. for 30minutes to produce a light sensitive body.

Parts by wt.

Photoconductive cadmium sulfide powder (used in Example 2) 100 AmyllacNo. 2 clear 1 42 Amyllac No. 2 thinner 200 Total 342 Butylated melamineresin, a paint vehicle for high temperature baking consisting ofnon-drying, short oil alkyd resin and epoxy resin, produced by KansalPaint 00., Ltd.

In the foregoing examples, cadmium sulfide powder is buried in the freesurface of the insulating photoconductive layer and because of this, thestrength of the coating is lower, as compared with a coating having nocadmium sulfide powder or a small content thereof. In order to eliminatesuch defect, a layer having no or small content of cadmium sulfidepowder may be applied to the surface layer to be 0.5-1 micron inthickness.

What is claimed is:

1. A process for the production of a light sensitive body having aninsulating photoconductive layer, which comprises dispersing a finepowder consisting of photoconductive cadmium sulfide having an averagegrain size of less than 1.5 microns in an electrically insulating,acrylic thermo-setting resin having a specific resistance by volume ofat least 10 .9 cm. at room temperature, applying the resultingdispersion to a support, and heating the support thus coated to atemperature of at least 150 C. for at least 30 minutes to harden theresin and simultaneously to carry out heat treatment of the fine powderof cadmium sulfide and resin.

2. The process according to claim 1 wherein the proportion of saidphotoconductive cadmium sulfide in said layer is from 20% to 60% byvolume.

3. The process according to claim 1 wherein the proportion of saidphotoconductive cadmium sulfide in said layer is from 32% to 42% byvolume.

References Cited UNITED STATES PATENTS 3,121,006 2/ 1964 Middleton eta1. 961.5 3,355,289 11/1967 Hall et al. 9-61.5

MURRAY KATZ, Primary Examiner US. Cl. X.R.

